The present invention relates to a color cathode ray tube having a shadow mask.
In general, a color cathode ray tube has an envelope. The envelope comprises a substantially rectangular panel having an effective portion constituted by a curved surface and a skirt portion provided on the circumference of the effective portion. It also comprises a funnel connected to the skirt portion. A phosphor screen is formed on the inner surface of the effective portion of the panel. The phosphor screen comprises light absorbing layers and phosphor layers of three colors buried in gaps between the light absorbing layers. A substantially rectangular shadow mask is arranged at a predetermined distance from the phosphor screen. An electron gun is mounted in a neck portion of the funnel.
Three electron beams emitted from the electron gun are deflected by a deflector mounted on the outer surface of the funnel, and scan the phosphor screen in the horizontal and vertical directions via the shadow mask, so that a color image is displayed.
The shadow mask comprises a rectangular mask body and a substantially rectangular mask frame to which a skirt portion of the mask body is attached. The mask body has a main surface constituted by a curved surface opposing to the phosphor screen and the skirt portion provided on the circumference of the main surface. The main surface has a number of electron beams passing apertures. The shadow mask is arranged inside the panel by engaging elastic supporting members attached to the mask frame with stud pins attached to the panel.
The side wall portion of the mask frame is parallel with the tube axis (the Z-axis) of the cathode ray tube and in contact with the skirt portion of the mask body. The edge of the side wall portion on the side of the phosphor screen has a shape corresponding to the curve of the circumference of the main surface of the mask body.
In the general shadow mask in which the main surface of the mask body opposing to the phosphor screen has curvatures in the directions of the long and short axes, a fall of the main surface, along the tube axis, at each end of a diagonal axis relative to the center of the mask body is greater than those at the ends of the long axis and the short axis of the main surface. Accordingly, the height of the side wall portion of the mask frame is lower at the ends of the diagonal axis than those at the ends of the long axis and the short axis.
A recent color cathode ray tube has a large size and the screen thereof is long sideways to have an aspect ratio of 16:9. In this type of color cathode ray tube, the main surface of the mask body has a large area. Further, to improve the visibility, the radius of curvature of the outer surface of the effective portion of the panel has been extended substantially infinitely large to flatten the panel. In this type of color cathode ray tube, the main surface of the mask body must also be flattened. In this case, the fall of the circumference of the main surface relative to the center of the mask body is small. Therefore, the mechanical strength of the shadow mask of the color cathode ray tube is low.
Generally, in the shadow mask, the mask frame has a greater thickness and a greater heat capacity as compared to the mask body. Particularly in a color cathode ray tube of large size, the mask frame must be thick to increase the mechanical strength of the shadow mask. Therefore, the difference in heat capacity between the mask body and the mask frame is much greater. In this case, the beam landing is deviated due to the difference in heat capacity, resulting in degradation of color purity.
The mask body is heated by impingement of electron beams, when the color cathode ray tube is operated. Since the heat generated in the mask body is transmitted to the mask frame via a contact portion therebetween, the temperature of the peripheral portion of the mask body is lower than that of the central portion thereof. Therefore, when the color cathode ray tube is operated, the central portion of the mask body is deformed more greatly as compared to the peripheral portion thereof. As a result, the electron beam landing on the phosphor layers of the three colors is displaced, so that the color purity is degraded.
Further, as multimedia applications have been developed, the arrangement pitch of the three-color phosphor layers in a color display tube used in a computer terminal or the like has become smaller than that of the ordinary color cathode ray tube. In this case, the margin of beam landing is small and color deviation easily occurs. For this reason, the beam landing is required to be more accurate in the color display tube.
Actually, the position of the shadow mask relative to the panel may be shifted due to shock which the color cathode ray tube receives in a manufacturing process or transport. Particularly in a large color cathode ray tube or a color cathode ray tube having the aspect ratio of 16:9, since the shadow mask is comparatively heavy, the position of the shadow mask relative to the panel is easily shifted due to shock in a manufacturing process or transport of the color cathode ray tube.
Further, in a large color cathode ray tube or a color cathode ray tube having the aspect ratio of 16:9, if the thickness of the mask frame is increased to mechanically strengthen the shadow mask, load on the elastic supporting members supporting the shadow mask is increased due to the increase in weight of the shadow mask. In this case, if the shock resistance is improved by, for example, increasing the elasticity of the elastic supporting members, in order to prevent the shadow mask from positional deviation due to shock, the mask frame may be deformed by force applied thereto when the shadow mask is attached to and detached from the panel. As a result, the position of the shadow mask relative to the panel is shifted.
The problem described above is particularly remarkable in the case of a color cathode ray tube with a flat panel in which the radius of curvature of the outer surface of the effective portion is substantially infinite.